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LASER TRIM CONTROL OF OSCILLATOR TEMPERATURE COMPENShTION

IP.com Disclosure Number: IPCOM000007507D
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2002-Apr-02
Document File: 2 page(s) / 109K

Publishing Venue

Motorola

Related People

Robert Bruce Sanders: AUTHOR

Abstract

An inherent problem in crystal oscillators is that the frequency will vary as a function oftemperature. One way to reduce this variation to an acceptable level is to use some sort of temperature compensa- tion scheme. In some applications the compensa- tion can be accomplished by the use of a TC cap in the oscillator circuit. The capacitance of a TC cap by definition will vary as a mnction of temperature. TC caps are available with various slopes. For exam- ple an NlOOO TC cap has a slope of- 1000 ppm/"C, while a P300 has a slope of +300 ppm/"C, and an NPO has a slope of 0 ppm/"C. Thus if a crystal were mated with a TC cap of the appropriate slope and installed in an oscillator, the temperature induced change in oscillator Frequency due to the crystal and the temperature induced change in oscillator fre- quency due to the TC cap will tend to negate each other. The end result is a more temperature stable oscillator.

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Technical Developments

LASER TRIM CONTROL OF ; OSCILLATOR TEMPERATURE COMPENShTION

by Robert Bruce Sanders

  An inherent problem in crystal oscillators is that the frequency will vary as a function oftemperature. One way to reduce this variation to an acceptable level is to use some sort of temperature compensa- tion scheme. In some applications the compensa- tion can be accomplished by the use of a TC cap in the oscillator circuit. The capacitance of a TC cap by definition will vary as a mnction of temperature. TC caps are available with various slopes. For exam- ple an NlOOO TC cap has a slope of- 1000 ppm/"C, while a P300 has a slope of +300 ppm/"C, and an NPO has a slope of 0 ppm/"C. Thus if a crystal were mated with a TC cap of the appropriate slope and installed in an oscillator, the temperature induced change in oscillator Frequency due to the crystal and the temperature induced change in oscillator fre- quency due to the TC cap will tend to negate each other. The end result is a more temperature stable oscillator.

  A common procedure for temperature compen- sation (TC) of oscillators used in paging receivers is to first sort the oscillator crystals based on how the crystal frequency varies over temperature. Each crys- tal is then individually marked to indicate the opti- mum slope TC cap to compensate that particular crystal. As each crystal is installed in the pager the proper slope TC cap must be installed with it. This presents a manufacturing complication since each

pager can not be built using a single identical set of parts. Any lot of crystals Will require the use of sev- eral different slope TC caps. It is not uncommon to use up to five distinct types of TC caps.

  Laser trim control of oscillator temperature com- pensation provides the same TC result while sim- plifying assembly. All units are built using an iden- tical set of parts. Two laser trimmable caps are connected in parallel to perform the function of a single non-tunable TC cap. One trim cap has a TC slope more positive thaniany effective TC slope that will be required. The other trim cap has a TC slope that is more negative than any effective TC slope that will be required. The effective TC cap slope is achieved by balancing the value of...